Integrated circuit technology has revolutionized various fields including computers, control systems, telecommunications, and imaging. For example, in the imaging field, CMOS image sensors have proved to be less expensive to manufacture relative to CCD imaging devices. Further, for certain applications CMOS devices are superior in performance. For example, the signal processing logic necessary can be integrated alongside the imaging circuitry, thus allowing for a single integrated chip to form a complete stand alone imaging device.
Despite the advances of CMOS image sensor technology, certain remaining problems prevent their widespread acceptance. One such problem is “kTC” noise introduced in CMOS pixels during a reset operation. Specifically, in CMOS arrays, each pixel must be reset before the pixel starts its “integration” period. Generally, this reset step requires that each pixel have an associated transistor switched on to allow a reset voltage to reach the photodiode of the associated pixel. When the associated transistor is turned off, the voltage of the photodiode should be equal to the reset voltage. However, thermal noise (referred to as kTC noise) in the transistor channel introduces some variability in the quantity of charge injected in each pixel after each reset. Because thermal noise is truly random, voltage variations on individual pixels occurring once will not necessarily occur with the same variation a second time. The magnitude of kTC noise is related to k, the Boltzmann constant, T, the temperature in Kelvin, and C, the capacitance in the current path. The local temperature variations in each pixel give rise to this random kTC noise.
Prior art attempts to combat kTC noise focused on varying the structure of the pixel. For example, pixels using four transistors have been proposed. However, this has the disadvantage of large pixel size and difficulty to manufacture. Similarly, in U.S. Pat. No. 5,981,932 describes a pixel that performs kTC noise canceling by storing charge in a pseudo-capacitor. However, this pixel requires additional process steps to manufacture, as well as additional timing requirements to operate. Further, the pixel of the '932 patent requires an additional transistor to provide additional switching to ground or power.
Thus, it is desirable to provide a simple and easy to manufacture method or apparatus that can compensate for kTC noise.